The infusion bag means a container which is used in injecting a liquid such as a blood preparation, drip infusion liquid and the like to a living body or in preserving them. Currently, glass make, plastic make and the like are used in the infusion bag, and a rubber stopper is inter-fitted to the opening of the container in order to prevent leakage of the liquid (containing liquid) filled therein. In using the infusion bag, it is general that an injection needle such as a metal needle, a plastic needle and the like is inserted into the rubber stopper and the containing liquid is pulled out through it.
In the above-mentioned use, the injection needle is pulled out from the rubber stopper at a period of time in response to each treatment, which is not only pulled out after complete consumption of the containing liquid but also pulled out under a state of remaining the containing liquid in the infusion bag. The characteristic required for the rubber stopper in that case is the liquid leakage sealing property. When this sealing property is insufficient, there is a case of causing leakage or scattering of the containing liquid from a hole formed where the injection needle is inserted.
Conventionally, a vulcanized rubber having excellent liquid leakage sealing property, such as an isoprene rubber, is frequently used in the rubber stopper for infusion bag, but not only the vulcanized rubber contains an additive agent which is not suited for articles for clinical use but also is inferior in terms of molding workability. In addition, while the main stream of infusion bag is from glass make to plastics make, it is possible to carry out fusion inter-fit with the main body of a plastic infusion bag made by a two-color molding, so that an attempt has been made to use rubber stoppers made of thermoplastic elastomers (Patent Document 1).
Since styrene-based elastomers are particularly excellent in rubber elasticity, these have been examined as the materials from which vulcanized rubber substitution is highly expected, and it has been disclosed that a styrene-based elastomer having a specified hardness range is excellent in the balance of liquid leakage sealing property, penetration strength and handling property (Patent Document 2).
Further, it has been disclosed that a styrene-based elastomer containing isoprene having a 1,4-microstructure expresses high liquid leakage sealing property (Patent Document 3). In addition, a technique of adding a petroleum resin has also been disclosed (Patent Document 4).